Recent episodes of forest stress and dieback are apparent on all
forested continents of the world. In particular, substantial episodes
of recent forest mortality have occurred in North America from Alaska
to Mexico, affecting more than 20 million hectares and many tree
species since 1997, a period of warming temperatures and significant
drought in many areas. Climate change models predict substantial
shifts in climatic patterns over coming decades in many regions,
including warmer temperatures and increases in duration and severity
of extreme drought events. Such changes increase stress on long-lived
woody vegetation, directly leading to increased mortality and
episodes of forest dieback. In some cases forest dieback is increased
even more by climate-mediated changes in populations of insect pests,
or human-altered land-use patterns and disturbances like forest
fragmentation and increased fire activity. Assessing the potential
for extensive climate-induced forest dieback is a key global change
research topic, since woody mortality losses can occur much faster
than tree growth gains, with pervasive and persistent ecological
effects, including feedbacks to other disturbance processes (e.g.,
fire, erosion) and loss of sequestered carbon back to the atmosphere.
In this session, USGS and over 20 researchers from around the world
present a synthesis of climate-induced forest dieback as an emergent
global phenomenon, including an international overview from ongoing
research and existing literature. Collectively the papers in this
organized oral session highlight global examples of forest dieback,
physiological process drivers of woody plant mortality, and
applications of available knowledge to regional and global scale
modeling and prediction of forest dieback. Craig D. Allen, Symposium
OOS 42 — Climate-induced forest dieback as an emergent global
phenomenon: patterns, mechanisms, and projections, Thursday, Aug. 9,
1:30-5:30 p.m.

After tracking the fates of more than 20,000 trees in a network of
old-growth forest plots in the Sierra Nevada of California for over
two decades (1983 – 2004), USGS scientists found that tree death
rates have increased significantly over the past 20 years. Death
rates increased not only for all trees combined, but also across most
elevational zones and for the two dominant groups of conifers, firs
and pines. The rising death rate for trees was paralleled by
increasing summer drought due to warming temperatures. These findings
suggest that Sierran forests, and potentially other forests of dry
climates, may be sensitive to temperature-driven increases in
drought, making them vulnerable to extensive die-back during
otherwise normal periods of reduced precipitation. Phil van Mantgem,
COS-142 — Climate change: physiological and population response,
Friday, Aug. 10, 10:10 a.m.
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